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Los Angeles recorded 298 murders in 2012, down a significant 39 percent from the 488 logged in 2005 , the LAPD and L.A. Mayor Antonio Villaraigosa reported Monday.

In general, serious crimes such as homicide, rape, burglary and theft were also down significantly: to 104,159 so-called Part I crimes in 2012, compared with 128,759 in 2005, a 19 percent decline.

During the same period, Los Angeles went from 9,284 police officers in 2005 to 10,023 last year, a big reason for the drop in crime, Villaraigosa and LAPD Chief Charlie Beck said.

"Cities in California that have stopped hiring or are cutting back on their police force have seen crime spikes," Villagairosa said. L.A. — once the national murder capital — has some of the lowest violent crime rates in the country, compared to other population hubs (and the lowest among cities over 2 million), he said.

Asked whether keeping the status quo would guarantee a continued relatively low crime rate, Beck replied, "absent other influences," that's a likely outcome. "If you apply more resources to crime, you reduce it."

True?

In a couple of decades, crime has plummeted in the United States. Criminologists offer a variety of reasons.

There's the increased incarceration rate that's taken large numbers of people out of circulation for years at a time (this is generally called the "incapacitation" effect of prison). Most criminologists say that accounts for about 25 percent of the national crime drop.

And there's a common theory that people have become more aware about protecting themselves from crime with burglar alarms and other pre-emptive measures.

Then there's a growing amount of research that points to medical reasons for dropping crime rates. A 2007 study by Amherst economist Jessica Wolpaw Reyes attributed half of the crime drop in the 1990s to lower lead levels in gasoline. Since then, multiple studies have established the link between lead exposure and criminal behavior.

And then there's police — their training, tactics and numbers. More and more, criminologists say advances in policing help to explain the drop in crime.

Perhaps the most comprehensive examination of this link was UC Berkeley Professor Franklin Zimring's book-length study on New York City's historic crime drop. As he researched his book, "The City that Became Safe," Zimring found that while crime declined around the country, New York City's decline eclipsed the nation by quite a bit — and its drop happened amid prison realignment in the state of New York. That significantly reduced the number of people in prison.

The crime drop, he found, had lots to do with changes in the way the city policed. Specifically, the size of the police force grew considerably, police started focusing more and more on "hotspots," and New York police began a vast (and highly controversial) stop-and-frisk campaign.

Los Angeles police chief Beck noted similar changes (minus stop-and-frisk) for the crime decline in L.A. Hot-spot policing, a religious devotion to shaping strategy through statistics, and the less-studied but highly respected anti-gang program (GRYD) of out of the Mayor's Office. As crime has started to tick up a bit in some cities, the decline, albeit small, continued in L.A. in 2012.

Clearly, policing isn't the only piece in the crime puzzle. Take a look at Chicago, which has one of the highest violent crime rates among big cities in the US (along with Philadelphia, a city whose gnarly homicide rate inexplicably gets less media and academic attention). The Chicago Police Department has 2,000-3,000 more police officers than L.A. and the city of Chicago about a million fewer people than L.A.

And in many pockets of L.A., violent crime remains a constant issue. In 2012, for instance, when homicide and violent crime rates reached historic lows, the 77th Street police district saw a 59 percent jump in murders (as of Dec. 29) from the previous year.

Correction: This story originally said Chicago has 200-300 more officers than L.A.